Composite restorative materials are finding ever-increasing use in dentistry. Although current composites are designed primarily for use as restorations for anterior and other non-load-bearing regions of tooth structure, these materials (perhaps prematurely) are being applied to the posterior acclusal region as well. As anterior restorative materials, many of the current composites have performed reasonably well, although clinical and other studies have shown that these esthetic filling materials have several significant shortcomings; namely, color instability, lack of stain resistance, marginal leakage and chemical erosion which contributes to anatomic loss.
The susceptibility of the organic matrix to chemical disintegration is a likely critical factor contributing to the wear of dental composites both in stress-bearing and stress-free applications. The complex interaction of composite restorations with the many chemical substances (e.g. H.sub.2 O, O.sub.2, food-related chemicals, etc.) found in the oral environment, can by diffusion-controlled processes initially lead to plasticization, and ultimately to chemical degradation of the polymer matrix. Since they exists in a predominantly aqueous environment, the transport of other chemical substances into the polymer phase is likely to be water-assisted.
The solubility parameter of the dental resin system employed for restorative materials, its water-related properties (e.g., water sorption), and its degree of polymerization and crosslinking, are important factors governing the extent of chemical softening and disintegration that will occur in dental composites. The relatively hydrophilic matrices of conventional bis-GMA (2,2,-bis[4'(3"-methacroyl-2"-hydroxypropoxy)phenyl]propane) or urethane methacrylate copolymer systems have solubility parameters similar to certain chemical substances (e.g. food derived chemicals) found in the oral cavity. Composites based on these resin systems, therefore, will display an affinity not only for water but also for many of the chemical moieties generated intraorally. The complex sorption/desorption processes that occur in these composites may induce not only stresses but also degenerative chemical reactions that accelerate the failure of these restorative materials.
U.S. Pat. No. 4,292,029 to Craig et al employs a large amount of fluorinated alkyl methacrylate (1,1,5-trihydrooctafluoropentyl methacrylate, OFPMA) in conjunction with an analog of bis-GMA in an attempt to overcome the drawbacks of the conventional bis-GMA resin system. The hydrophobic composite represents a significant advance in designing an esthetic restorative material with enhanced resistance to the assaults of the oral environment. The dental resin system suffers however from several serious deficiencies, namely (a) loss of OFPMA on mixing because of its relatively high vapor pressure and its greater potential for irritation than a bulky fluorinated monomer of low vapor pressure; (b) undesirably greater opacity than desirable because of the relatively low refractive index of OFPMA and (c) low strength.
The relatively low crosslinked density of the OFPMA polymeric matrix results in composites of relatively low strength which fail to meet the minimum requirement for diametral tensile strength (34 MPA) established by the American Dental Association. In addition, such composite materials exhibit a relatively high contraction on polymerization. Craig mentions but does not give actual examples of composites derived from fluorinated bis-GMA analogs or other fluorinated dimethacrylates (e.g. tetrafluoroethylene glycol dimethacrylate); the existence of the latter is doubtful. Also, the use of fluorinated silane agents is mentioned, but again, no examples of how they are used are given. The fluorinated silane agents (e.g. hydrooctafluorobutyltrichlorosilane) cited are actually not coupling agents and their sole or improper use leads to extremely weak composites. Only the dual silanization procedure disclosed below leads to strong composites having excellent compatibility of the resin and filler components.